The invention relates to a digital echo canceller with a receive path between a receive input and a receive output, and a send path between a send input and a send output, said echo canceller being used for cancelling an additive echo signal at the send input which has occurred in response to a receive input signal applied to the receive input, said echo canceller including:
a filter combination comprising PA0 means for forming a send output signal as the difference between the signal applied to the send input and the first replica signal; PA0 controllable gate means for selectively applying the modified filter coefficients to the programmable filter coefficient memory of the first digital filter; and PA0 control means for blockwise determining the respective levels of the error signal and the send output signal, and for generating in response to the levels thus found a control signal for the gate means which depends in a predetermined manner on differences between the respective levels. PA0 in the said filter combination the second digital filter is a frequency-domain block-adaptive filter having a block length of N' components and having, for each signal block m, a number of N' frequency-domain filter coefficients W(p;m) with p=0,1,2, . . ., N'-1, and the first digital filter is a time-domain programmable digital filter having a number of N time-domain filter coefficients W(i;m) with i=0,1,2, . . . , N-1, with N' exceeding N; and PA0 the echo canceller further includes transforming means, cascaded with the gate means, for forming the N time-domain filter coefficients w(i;m) required in the first digital filter as components of an N'-point Discrete Orthogonal Transform of a block of N' frequency-domain filter coefficients W(p;m). PA0 a considerable reduction of the number of computational operations required for cancelling echo signals; this especially plays a role in echo paths having an impulse response of a large length, such as acoustical echo paths in which a relatively large number (1000 to 2000) of filter coefficients is required for the echo cancellation; PA0 a speed of convergence that can be increased in a simple manner; PA0 an echo cancellation with a negligible delay; and PA0 a slight bottom noise despite the absence of window means in the FDAF. PA0 the echo canceller also comprises a cascade arrangement of second controllable gate means and second transforming means for forming the N' frequency-domain filter coefficients W(p;m) as components of a Discrete Orthogonal Transform of the N filter coefficients w(i;m) in the programmable filter coefficient memory of the first digital filter and for selectively feeding back the thus formed filter coefficients W(p;m) to the second digital filter; and in that PA0 the control means are also arranged for blockwise determining the respective levels of the receive input signal and the send input signal, and for generating a second control signal for opening the second gate means if the level of the receive input signal thus determined is lower than the level of the send input signal thus determined.
a first digital filter with a programmable filter coefficient memory for generating in response to the receive input signal a first replica signal which is an estimate of the echo signal, and PA1 a second, adaptive digital filter with a filter coefficient memory for generating in response to the receive input signal a second replica signal which is an estimate of the echo signal, the second digital filter further including means for forming an error signal that is representative of the difference between a signal applied to the send input and the second replica signal, and an adaptation processor for adaptively modifying filter coefficients in response to the receive input signal and the error signal and applying the modified filter coefficients to the filter coefficient memory of this second digital filter;
A digital echo canceller of such a structure is known from an article entitled "Echo Canceler with Two Echo Path Models" by K. Ochiai et al., published in IEEE Transactions on Communications, Vol. COM-25, No. 6, June 1977, pp. 589-595.
The echo canceller described in this article is especially arranged for combatting the disturbing effect on the echo canceller adjustment caused by double-talk. Double-talk occurs when a desired signal to be transmitted and an echo signal are simultaneously applied to the send input. The superpositioning of these signals then results in that the adjustment of the echo canceller for cancelling the echo signal can be deranged significantly by the signal to be transmitted that is also present. This means that the current echo signal is then no longer cancelled sufficiently by the replica formed by the echo canceller. In the above article a robust solution is given to the problem of a possible misadjustment of the echo canceller caused by double-talk. For this solution a filter combination is used consisting of a first digital filter which comprises a programmable filter coefficient memory and which is used for the echo cancellation proper, as well as a second, adaptive digital filter with an associated filter coefficient memory. These two filters each generate a replica of the echo signal and as long as the replica generated by the adaptive filter is a better estimate of the echo signal than the replica generated by the programmable filter the filter coefficients of the adaptive filter are transferred to the programmable filter. During double-talk the adjustment of the adaptive filter is disturbed and then the transfer of filter coefficients to the programmable filter is interrupted. This achieves that the adjustment of the adaptive filter does not disturb the operation of the programmable filter for the echo canceller proper during double-talk.
This known digital echo canceller is implemented completely in the time-domain. In the field of speech and data transmission, most applications utilize time-domain adaptive filters (TDAF) that are realized as adaptive transversal filters using a least mean square (LMS) algorithm for the adaptation of filter coefficients. When the length of the impulse response of the echo path assumes large values, such as, for example, can be found in applications in the field of acoustics, a TDAF realized as a transversal filter presents the drawback that the complexity expressed in terms of computational operations (multiplications and additions) per output sample increases linearly with the number of discrete-time components with which the impulse response of the echo path can be represented. More specifically, for this known echo canceller it holds that the number of operations required for computing N components is in the order of magnitude of N.sup.2 for the programmable filter with respect to the calculation of N samples of the first replica signal, and in the order of magnitude of N.sup.2 for the adaptive filter with respect to the calculation of N samples of the second replica signal and also in the order of magnitude of N.sup.2 with respect to the calculation of adaptations for N filter coefficients. In addition, a TDAF realized as a transversal filter has a low convergence speed for strongly (auto) correlated input signals, such as speech and specific kinds of data. This is because the convergence speed decreases as the ratio between maximum and minimum eigenvalues of the correlation matrix of the input signal increases. In this connection reference is made to an article entitled "Echo Cancellation Algorithms" by C. W. K. Gritton and D. W. Lin, published in IEEE ASSP Magazine, April 1984, pp. 30-38, more specifically the section "LMS Algorithm" on pp. 32-33.